In many surgical operations it is necessary to obtain direct access to the cranial cavity and the brain. To perform such operations it is often necessary to drill holes through the skull bone. Since the bone is very hard, it is necessary to apply significant pressure to drill through it. Since the dura beneath the skull bone and the brain itself are very delicate, it is important that the drilling cease immediately before the dura is cut or damaged.
In the past, surgeons have used hand braces and bits of a design very similar to those used for non-medical purposes, for example carpentry. Such tools are not completely satisfactory because it has been found that such tools can cut through the skull and damage the meninges or brain and tend not to leave the skull or the underlying membranes in a condition that enables them to heal to approximately their original condition. It has also been found that hand tools are slow and require the surgeon to exert a great deal of energy. Attempts have been made in the past to provide power driven cranial drills which remedied some of these conditions. U.S. Pat. No. 2,842,131, entitled Automatic Drill, by G. W. Smith, discloses a cranial drill. The Smith drill includes a primary drill which is slideable and guidable freely in a tapered safety counterbore collar. The Smith drill includes a clutching mechanism which permits the primary drill to automatically release once the last shelf of the inner table of the skull structure is removed at the base of the opening. The counterbore, made by the counterbore collar, provides a support for the drill mechanism so that the primary drill may release when it penetrates the skull without having the remainder of the drill moved in the direction toward the cranial cavity.
The cutting surfaces of the Smith drill are designed to remove the bone structure in large, generous bone chips so that the chips can be replaced and reincorporated into the whole structure after the operation is performed.
The Smith drill is designed to be dismantled for cleaning and sterilizing. It has been found, however, that the cleaning and sterilizing process can be expensive because skilled personnel must spend significant amounts of time reassembling and testing the drill after cleaning and sterilizing.
Thus, there is a need for a completely disposable, permanently assembled drill, which cannot be dismantled, cleaned, sterilized, reassembled, and periodically sharpened and which includes an indicator to advise the user whether or not attempts have been made to clean or sterilize the drill.
The clutch mechanism in the Smith drill uses a spring force to assist in the disengagement of the drill mechanism from the driver. A cam action provided by the interaction of the component parts of the Smith clutch also provides an axial force on the Smith primary drill to further assist in the disengagement of the clutch as the drill penetrates the bone structure being drilled. It is desirable to have the clutch mechanism disengage as quickly as possible after the primary drill penetrates the skull.